This invention relates to a wafer cleaning technology and, more particularly, to a method for cleaning a semiconductor wafer after a chemical mechanical polishing on a copper wiring.
One of the technical goals for a semiconductor logic circuit device is a high-speed logic function. A copper wiring is attractive, because the electric resistance is low and the anti-electromigration property is good. The copper wiring is employed in the semiconductor logic circuit device. Although the copper wiring is desirable in view of the electric resistance and the anti-electromigration property, a copper layer is hardly patterned through a dry etching. The copper wiring is usually patterned as follows. First, a groove is formed in an insulating layer, and barrier metal such as titanium or tantalum and copper are successively deposited over the entire surface of the insulating layer. The entire surface of the insulating layer is covered with the barrier metal, and defines a secondary groove in the groove. The copper fills the secondary groove, and swells into a copper layer spread over the upper surface of the barrier metal layer. The copper layer and the barrier metal layer are chemically mechanically polished until the insulating layer is exposed, again. The copper is left in the secondary groove, and forms the copper wiring.
Polishing slurry is used in the chemical mechanical polishing, and contains polishing particles. Copper particles are produced during the chemical mechanical polishing. For this reason, when the copper wiring is completed, various kinds of contaminant are left on the surface of the insulating layer and the surface of the copper wiring. If the fabrication process is continued without any surface cleaning, the copper particles and other metallic contaminant such as Fe, Ca and K are diffused through the insulating layer, and reach the circuit components. The contaminant deteriorates the circuit components. Thus, the surface cleaning is indispensable, and is an important technology in the fabrication process.
The copper wiring is not limited to the semiconductor logic circuit device. Logic circuits are integrated on a semiconductor chip together with other function blocks such as, for example, a dynamic random access memory block, and the copper wiring is employed in those semiconductor integrated circuit devices.
The present inventors searched documents already published for a surface cleaning after the chemical mechanical polishing on the copper layer. However, the present inventors could not find any surface cleaning. For this reason, a surface cleaning after a chemical mechanical polishing on a tungsten player is hereinbelow described.
Titanium or titanium nitride is used as a barrier metal layer for the tungsten wiring, and the tungsten is deposited over the barrier metal layer. While a tungsten layer is being chemically mechanically polished, polishing slurry is supplied to the polishing pad. The polishing slurry contains alumina particles or silica particles dispersed in mixture of hydrogen peroxide and oxidizer. Upon completion of the chemical mechanical polishing, the alumina particles or the silica particles are left over the silicon wafer at 3000 particles per wafer. The tungsten powder is also left on the silicon wafer.
A known surface cleaning after the chemical mechanical polishing uses diluted hydrofluoric acid, and another known cleaning technology uses a brushing in aqueous ammonia and a centrifugal spray cleaning (see MRS. Symp., Proc. Vol. 386, pages 109 (1995) written by I. J. Malik et. al.) using citric acid. However, the diluted hydrogen peroxide is known as a kind of etchant. The titanium and the titanium nitride are etched in the diluted hydrofluoric acid at large etching rate. For this reason, the diluted hydrofluoric acid is not appropriate for the surface cleaning after the chemical mechanical polishing on the tungsten wiring laminated on the barrier metal layer.
The second prior art surface cleaning is available for the tungsten wiring. The aqueous ammonia contains the ammonia at 0.1-2 weight percent, and the particles are brushed away in the aqueous ammonia. Thereafter, the wafer is spun, and the citric acid is sprayed onto the tungsten wiring for removing the metallic contaminant.
It is therefore an important object of the present invention to provide a cleaning method after a chemical and mechanical polishing for a copper wiring without undesirable etching.
The present inventors evaluated the prior art cleaning technologies to see whether or not any one of them was available for a copper wiring. The present inventors prepared samples of copper wiring. Copper was formed on a barrier metal layer of titanium/titanium nitride through a metal plating and a chemical vapor deposition. The present inventors carried out the chemical mechanical polishing on the copper layers. Upon completion of the chemical mechanical polishing, the present inventors measured the copper contaminant. The copper contaminant was equal to or greater than 1012 atoms/cm2.
The samples were cleaned through the prior art methods. The diluted hydrofluoric acid etches the barrier metal layer at a large etching rate, and was not available for the post cleaning. The present inventors brushed the samples in the aqueous ammonia. Although the aqueous ammonia removed the particles from the copper wiring, a large number of pits were produced in the exposed surface of the copper wiring. The aqueous ammonia reacted with the copper so as to produce ammonia complex, and the ammonia complex was dissolved. The copper was polycrytal, and the aqueous ammonia penetrated along the grain boundaries. This meant that the ammonia complex was produced along the grain boundaries, and the copper grains were separated. The citric acid did not etch the barrier metal layer. However, the citric acid at high concentration was required for removing the metallic contaminant such as Fe. The high-concentration citric acid increased the cleaning cost. The citric acid was an organic compound, and the amount of total carbon in the wastewater was not ignoreable. Thus, the second cleaning technology was also not available for the copper wiring.
In accordance with one aspect of the present invention, there is provided a method for cleaning a semiconductor wafer having a copper layer exposed to a surface thereof after a chemical mechanical polishing comprising the steps of a) treating the semiconductor wafer with a first washer selected from the group consisting of aqueous ammonia containing ammonia at 0.0001-0.5 weight percent, a catholyte between neutral and weak base and hydrogen containing water for removing polishing particles from the semiconductor wafer without damage to the copper layer and b) treating the semiconductor wafer with a second washer containing at least one decontaminating agent selected from the group consisting of polycarboxylic acids each capable of producing a chelate compound together with copper, ammonium salts of the polycarboxylic acids and polyaminocarboxylic acids for removing metallic contaminants from the semiconductor wafer.
In accordance another aspect of the present invention, there is provided another method for cleaning a semiconductor wafer having a tungsten layer exposed to a surface thereof after a chemical mechanical polishing comprising the steps of a) treating the semiconductor wafer with a first washer selected from the group consisting of aqueous ammonia containing ammonia at 0.0001-5 weight percent, a catholyte between neutral and weak base and hydrogen containing water for removing polishing particles from the semiconductor wafer without damage to the tungsten layer and b) treating the semiconductor wafer with a second washer containing at least one decontaminating agent selected from the group consisting of oxalic acid, ammonium oxalate and polyaminocarboxylic acids and ranging from 0.01 weight percent to 7 weight percent for removing metallic contaminants from the semiconductor wafer.